1. Field of the Invention
The present invention relates to a method for repairing a photomask by removing a residual defect in the photomask and particularly to a method for repairing a phase shift photomask by removing a residual defect in the photomask.
2. Background Art
In recent years, an increase in integration density in a semiconductor integrated circuit has resulted in an ever-increasing demand for an increase in fineness also in a reticule used in the preparation of this circuit. For example, for DRAM, the line width of a device pattern transferred from a reticule for 16 M DRAM is as small as 0.5 .mu.m. Further, a device pattern of 64 M DRAM requires a resolution of a line width of 0.35 .mu.m. The conventional light exposure system using a stepper has reached the limit of the ability to provide a further increased fineness.
This inability has led to studies on various methods that can increase the resolution of the device pattern to a level usable for practical use.
Among others, a phase shift photomask that uses the conventional stepper exposure system and can increase the resolution of the device pattern transferred from the reticule has attracted attention.
Various phase shift photomasks have been proposed in the art, and, of them, a Levenson phase shift photomask (alternating phase shift mask) shown in FIG. 11 (a) and an attenuated phase shift photomask shown in FIG. 12 (b) have been put to practical use.
For the photomask shown in FIG. 12 (a), a light-shielding layer having a repeated pattern is provided on a transparent substrate, and a shifter layer 631, which functions to change the phase of the exposing light to half-wavelength, is provided in every other space section among space sections between adjacent light-shielding layers. On the other hand, for the photomask shown in FIG. 11 (a), a transparent substrate 610 is engraved to change the phase of exposing light to half-wavelength, and this type of photomask is known as "substrate engraving type." In this case, when a quartz substrate is used as the transparent substrate, the photomask is known as "quartz engraving type." FIG. 11 (b) shows a "Cr on shifter" type photomask wherein an SiO.sub.2 shifter layer 631 is provided under the light-shielding layer 621 to change the phase of the exposing light by a half-wavelength. FIG. 11 (c) shows "shifter on Cr" type photomask wherein an SiO.sub.2 shifter layer 632 is provided on the top of the light-shielding layer 622 to change the phase of the exposing light by half-wavelength.
FIG. 12 shows a photomask wherein a halftone pattern is provided and the half-wavelength change of the phase is performed by utilizing a part of the halftone section, wherein FIG. 12 (a) shows a photomask having a halftone layer having a single layer structure of a nitride oxide layer or the like and FIG. 12 (b) shows a photomask having a halftone layer having a multilayer structure of a nitride oxide layer 654 and a nitride layer 664 or the like.
Residual defects, which partially deteriorate the exposing light transmittance of the mask, are frequently created in the course of the production of the above phase shift photomasks.
The presence of residual defects in the mask makes it impossible to prepare a desired pattern by the transfer. Therefore, the photomask with residual defects has been repaired by physically or chemically removing the residual defect area to recover the light transmittance of the photomask in its residual defect area and an area around the residual defect area. A demand for an increase in fineness of the device pattern, however, has lead to a demand for removal of defects having a smaller size and a further improvement in accuracy of the repair of the photomask.
For repairing the photomask by removing a residual defect in the photomask, a laser beam irradiation repair method has been used wherein the defect area is evaporated and removed by a laser beam, such as YAG.
In this laser beam irradiation repair method, a laser beam, which has been widened to some extent and adjusted to predetermined form (usually in a rectangular form), size and intensity distribution by means of an aperture, is further concentrated and focused after passage through the aperture and then applied to the defect area to be removed. The size of the laser beam irradiation area may be varied by varying the size of the aperture so as to cope with various defect sizes and shapes.
Due to maladjustment of the aperture, maladjustment of focus, maladjustment of optical axis and the like, the shaped laser beam applied is not even in its intensity at the edge thereof, generally resulting in nonlinear and rough edges in the repaired end portion and often resulting in "roll-up" of the light-shielding layer of the photomask.
Further, when the defect area is evaporated and removed by a laser beam, a part of the underlying transparent substrate (usually a quartz substrate) also is evaporated to a depth of about 10 to 50 .ANG., creating roughening of the repaired area.
Although the roughened repaired area poses no particular problem in the case of exposure using an i-line stepper, it becomes a problem in the adoption of shorter wavelength light in the exposure at the time of transfer of the photomask. For example, the roughness of the repaired area is a severe problem in the transfer using an excimer laser as the light source for the exposure.
Further, the laser beam has focusing limits due to diffraction. This limit is generally regarded to be about 0.5 .mu.m.
At the present time, in 16 M DRAM, a residual defect having a size of 0.5 .mu.m is inspected and removed. This corresponds to the limit of the laser beam focusing ability. In 64 M DRAM, repair of the photomask on a level beyond the laser beam focusing ability is required.
On the other hand, instead of the repair using the laser beam, a method for removing a residual defect by means of a focused ion beam has become adopted as means which could cope with the demand for an increase in the fineness of the device pattern.
This method, however, suffers from a problem that the repair area cannot be limited to only the defect area due to irregular defect shape, often leading to damage to the transparent substrate (quartz glass or the like) in its area around the defect. An additional problem involved in this method is that gallium commonly used as an ion beam is unfavorably implanted into the underlying transparent substrate, creating a phenomenon called "gallium stain," which results in lowered light transmittance of the photomask in its repaired area.
In recent years, in order to reduce the gallium stain or the like, gas assisted etching is being put to practical use. In this method, excitation is performed by means of gallium ion to selectively etch the defect area alone. Even this method, however, cannot recover the light transmittance perfectly. Therefore, it is needless to say that recovery of the light transmittance cannot be achieved by the above method in the case of shorter wavelength light in the exposure.
As described above, the demand for an increase in fineness of the device pattern has lead to an ever-increasing demand for removal of defects having a smaller size and a further improvement in accuracy of the repair of the photomask. For this reason, various types of maladjustments, focusing limit of the laser beam, and creation of roughening in the repaired area involved in the repair of the photomask by the conventional laser beam irradiation, and damage to the transparent substrate (quartz glass or the like) and gallium staining involved in the repair of the photomask by the focused ion beam irradiation have become more and more severe problems.